1. Field of the Invention
This invention relates to compact disc (CD) player systems, and more particularly, to a CD player system with vibration-immune uninterrupted playback capability that can prevent interrupt in the audio or video playback in the event of the occurrence of any external vibrations that cause an interrupt in the data reading operation on the optical disc due to defocusing and mistracking of the laser pickup head of the CD player system.
2. Description of Related Art
A CD (compact disc) is an optical disc used to store digital information, such as audio, video, or computer programs and data, characterized in the large storage capacity compared to conventional magnetic storage means. The data stored on a CD can be read out by a laser pickup means. When used to store computer programs and data, it is usually referred to as a CD-ROM (compact-disc read-only-memory). A CD-ROM drive is an input device for personal computers which can handle various kinds of CDs, including video CDs, audio CDs, and CD-ROMs, while a CD player is a electronic device that is capable of reproducing audios from an audio CD.
In the use of a CD-ROM, it is only required that the computer programs or data on the CD-ROM be read out and loaded into the memory of the computer quickly. A short interrupt to the data reading operation on the disc is usually unperceivable by the user. However, in the use of a CD player for playback of audio data, such as musical pieces, a short interrupt to the data reading operation on the disc may cause an interrupt to the audio playback, which is usually intolerable to music lovers.
Interrupts to the data reading operation on the optical disc are usually caused by external vibrations, in that external vibrations can cause jumps--and thereby defocusing and mistracking--to the laser pickup head used for data reading operation on the optical disc. In the case of reading a CD-ROM, such interrupts will only cause a slightly prolonged access time and nothing to the integrity of the programs or data being read out.
In the case of reading an audio or video CD, however, such interrupts will cause intermittent breaks in the continuity of the audio or video playback, which is very unpleasing to the watching eyes and listening ears. This drawback is particularly apparent for CD player systems which are installed on automobiles, in that a running automobile can cause large vibrations to the CD player systems.
One solution to the foregoing problem is to provide a memory buffer to the CD player system. All the playable audio data will be first forwarded to and stored in this memory buffer before they are forwarded to the audio reproduction means. During the time of the occurrence of an interrupt to the data reading operation on the optical disc caused by a defocusing or mistracking of the laser pickup head due to external vibrations, the current contents in the memory buffer can be still used for playback for a short time until the laser pickup head resumes the data reading operation. The following will give a pictorial description of the foregoing conventional CD player systems and their drawbacks in more details.
FIG. 1 shows a schematic block diagram of a typical conventional CD player system used for playback of an optical disc 10. This CD player system includes a spindle motor 20, a laser pickup head 30, a radio frequency (RF) amplifier 40, a servo circuit 50, a data processor 60, a microcontroller 70, a digital-to-analog (D/A) converter 80, an amplifier 90, and a loudspeaker 100. The spindle motor 20 is used to spin the optical disc 10 at a fixed speed during playback operation to allow the data stored on the optical disc 10 to be read out by the laser pickup head 30.
The data signal from the laser pickup head 30 is first amplified by the RF amplifier 40. The output of the RF amplifier 40 is split into two parts: a first part of the amplified signal is transferred to the servo circuit 50 and a second part of the same is transferred to the data processor 60. The signal transferred to the servo circuit 50 is used for feedback controls of both the speed of the spindle motor 20 and the focusing and tracking of the laser pickup head 30. These feedback controls are conventional techniques well known in the art and not within the spirit of the invention, so details thereof will not be further described herein.
The amplified signal transferred to the data processor 60 then undergoes various data processing stages, such as eight-to-fourteen modulation (EFM) decoding, subcode acquisition, error correction and detection (ECD), and so on, to thereby obtain the playable audio data. Since the playable audio data are in digital form, they are further converted by the D/A converter 80 into analog form. The output of the D/A converter 80 is then amplified by the amplifier 90 for audible reproduction by the loudspeaker 100. Moreover, the microcontroller 70 can accept external commands from the user for various controls of the playback.
In the CD player system of FIG. 1, the laser pickup head 30 is a mechanical component which can be easily subjected to external vibrations that would cause defocusing and mistracking to the laser pickup head 30. Should this happen, the data reading operation on the optical disc is interrupted, thus causing an interrupt to the audio playback. This drawback is particularly apparent for CD player systems installed on automobiles which would cause large vibrations when running.
FIG. 2 shows a solution to the foregoing problem, in which an additional RAM unit 110 is coupled to the data processor 60 in the CD player system of FIG. 1. In the CD player system of FIG. 2, all the output data from the data processor 60 will be first forwarded to the RAM unit 110 and temporarily stored therein before they are forwarded to the audio reproduction means (the D/A converter 80, the amplifier 90, and the loudspeaker 100). In the event that the data reading operation on the optical disc 10 by the laser pickup head 30 is interrupted due to external vibrations, the current contents in the RAM unit 110 can be supplied to the audio reproduction means for uninterrupted playback until the laser pickup head 30 resumes data reading operation. By using this solution, however, it is required that the RAM unit 110 be always filled with enough playable audio data from the data processor 60 that can be fetched for playback during the period the data reading operation on the optical disc 10 is interrupted. The microcontroller 70 will command the spindle motor 20 to speed up the spinning of the optical disc 10 when vibrations occur, and speed down the same when the RAM unit 110 is about to overflow.
Therefore, with the CD player system of FIG. 2, the audio playback would not be interrupted when the CD player system is subjected to external vibrations that cause defocusing and mistracking to the laser pickup head 30 and thereby an interrupt in the data reading operation on the optical disc 10. The microcontroller 70 helps to keep the RAM unit 110 filled with an appropriate "backup" amount of playable audio data by controlling the speed of the spindle motor 20.
In the foregoing CD player system of FIG. 2, it is required that, after an interrupt has occurred, the resumed data reading operation start from the interrupted point (i.e., the next byte of audio data following the last byte that has been successfully read out just before the interrupt occurs).
Related patents of CD player systems include the U.S. Pat. No. 5,379,284 "COMPACT DISC PLAYER HAVING AN INTERMEDIATE MEMORY STORAGE DEVICE" to D. G. King, which uses an intermediate memory storage device to keep the audio playback uninterrupted in the event of the CD player system being subjected to external vibrations. Moreover, the U.S. Pat. No. 5,148,417 "TRACKING JUMP COMPENSATOR FOR OPTICAL DISC REPRODUCING APPARATUS" suggests the use of a compensator to keep the audio playback uninterrupted in the event of the CD player system being subjected to external vibrations. These two patents, however, fail to keep the resumed data reading operation to start precisely from the interrupted point.
To solve the foregoing problem, the U.S. Pat. No. 5,615,194 "DISK REPRODUCING APPARATUS AND METHOD FOR REPRODUCING AN AUDIO SIGNAL AND SUBCODE AT N TIMES NORMAL SPEED FROM A DISK" to I. Kimura discloses an apparatus and method which can achieve the so-called one-to-one correspondence of data on the optical disc to keep the resumed data reading operation after an interrupt has occurred to start precisely from the interrupted point. To achieve uninterrupted playback, the disclosed apparatus stores the audio data and associated subcode in a temporary memory unit and then keeps monitoring the subcode from the EFM encoder.
FIG. 3A is a schematic diagram used to depict a standard data format for storing audio data on an optical disc. As shown, the audio of data of a single musical piece, for example, is divided into a plurality of segments, each of which is preceded with a synchronization code SYNC, as designated by the reference numeral 200, and a subcode, as designated by the reference numeral 210, following the SYNC code 200. Typically, the subcode 210 consists of eight channels, which are usually referred to by the letters P, Q, R, S, T, U, V, and W, respectively. A total of 98 subcodes constitute one data frame.
FIG. 3B is a schematic diagram used to depict the structure of each subcode. As shown, the subcode includes eight channels P, Q, R, S, T, U, V, and W, each of which begins with two specific pieces of code data S0 and S1. FIG. 3C shows the contents of channel Q, which include such data fields as (Min: Sec: Frame) and (A.sub.-- MIN, A.sub.-- SEC, A.sub.-- FRAME) which can help the CD player system to resume the data reading operation on the optical disc, after an interrupt has occurred due to vibrations, to start precisely from the interrupted point. For details of this, please refer to the specification of U.S. Pat. No. 5,615,194.
One drawback to the foregoing patent, however, is that, it requires the temporary memory unit to have a large capacity to store the subcode data in addition to audio data. Moreover, it requires quite a complex hardware circuit to implement. These two drawbacks cause the implementation cost of the patent to be very high.
Conventional solutions to the problem of interrupt in the data reading operation on the optical disc due to external vibrations are thus still unsatisfactory. There exists, therefore, a need for a new CD player system which allows the resumed data reading operation on the optical disc, after an interrupt has occurred due to external vibrations, to start precisely from the interrupted point without losing any bytes of audio data between the newly located byte and the last byte that has been successfully read out just before the interrupt occurs.